Improvements in integrated circuit (IC) design, materials and manufacturing technologies now permit the manufacture of Very Large Scale Integrated (VLSI) circuits containing hundreds of thousands of functional circuit elements. As these technologies continue to develop, higher levels of integration and greater circuit densities are expected. While higher levels of integration provide many significant advantages, e.g. reduction of product costs, reduced product weight and size, increased product sophistication, lower energy consumption and operating costs, increased reliability, etc., the functional testing of VLSI circuits has become an increasingly complex process. Testing costs are becoming a significant portion of total IC manufacturing costs.
Several methods have been developed to simplify and reduce the cost of testing integrated circuits (ICs). One method utilizes a pseudo random pattern generator (PRPG) to generate test patterns which are applied to the IC under test. The PRPG, unlike a binary counter, produces a succession of binary test patterns wherein the ratio between binary ones and binary zeros is 1:1 for a substantial number of successive test patterns. The use of pseudo random test patterns considerably reduces the number of patterns needed to test a device.
Another testing method applies a weighted random test pattern to the IC under test. This procedure provides a statistically predetermined greater number of binary ones or binary zeros to the IC under test. Emphirical studies have shown that weighted random pattern testing can significantly increase test pattern coverage to maximize the effect on the IC internal circuitry when compared with unweighted random pattern testing.
A further discussion of pseudo random pattern testing and weighting is provided by Eichelberger et al. in U.S. Pat. No. 4,801,870.
Further simplification and reduction in costs are obtained by implementing testing techniques through the use of Built-In Self-Test (BIST) circuits fabricated into VLSI circuit chips during manufacture. A BIST test access port and boundary scan architecture for digital integrated circuits is described in IEEE Standard 1149.1, incorporated herein by reference. This standard defines the design and operation of various design-for-test features built into ICs and sets forth a standard instruction set for executing self-test functions.
Although the test methodologies and BIST structures developed in recent years have simplified testing of integrated circuits, there exists a continuing need for improved testing techniques as IC circuit densities and levels of integration continue to increase.